<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .a3 {
        height: 200px;
        width: 200px;
        border: 1px solid black;
        background-color: gold;
        margin: 30px auto;
    }

    .a3>.a4 {
        width: 120px;
        height: 50px;
        background-color: green;
        margin: 75px auto;
        
    }
    .a5 {
        
        width: 120px;
        /* 内容的宽 */
        height: 50px;
        background-color: gold;
        padding: 75px 40px;
    }
    .a5>.a6 {
        width: 120px;
       height: 50px;
        background-color: green;
        
    }
</style>

<body>
    <div class="a3">
        <div class="a4">

        </div>
    </div>
<div class="a5">
<div class="a6">

</div>

</div>

</body>

</html>